Aviation is an inherently risky mode of travel in bad weather. Unlike other modes of transportation such as by land or sea where dangerous weather conditions can be avoided by “waiting out the storm”, an aircraft carries a finite amount of fuel to power its engines, and must land at a suitable airport before it runs out of fuel. In addition, the great distances that an aircraft can travel in a relatively short period of time allow for drastically different landing conditions from takeoff to landing. In today's society where time is a valuable asset, pilots often feel pressured to land their aircraft in weather conditions they might otherwise wish to avoid. In order to remain safe, pilots must be provided with very accurate information regarding real-time aircraft performance during landing in those conditions.
Most pilots currently rely partly on information provided by another pilot who has recently landed for a description of the runway conditions. A contaminated runway may present hazardous conditions for landing aircraft because the effectiveness of wheel brakes may be reduced or negated by a low coefficient of friction (resulting from the ground contamination) between the runway surface and the wheels of the landing aircraft. Even so, individual pilot experiences are different and reports of this kind are very subjective. For a pilot who regularly lands in winter conditions, a little snow on the runway may be perceived and reported as good landing conditions, whereas a pilot who has mostly or always landed on non-snow covered, or otherwise contaminated runways may think that the same runway is in poor condition. The subjective nature of these observations limits their usefulness.
Another technique to provide pilots with runway characteristics is the use of a ground vehicle with a friction measurement device. Several of these devices have been conceived using different techniques to measure the current frictional characteristics. Many of these devices (for example U.S. Pat. No. 4,144,748) use a fixed slip ratio between the rotational speed of the measurement wheels and ground speed. This fixed slip produces a force proportional to the frictional characteristics of the surface being driven upon, and can give an indication of the condition of the runway. This measurement is difficult to interpret by an airplane pilot flying an aircraft with a varying and modulating slip ratio as in an aircraft anti-skid braking system (ASBS), and thus has limited applicability to assist the pilot in safely landing their aircraft.
Other devices such as U.S. Pat. Nos. 4,958,512, 5,814,718 and 6,711,935 use variable braking of the measurement wheel in an attempt to locate the peak frictional coefficient for the current runway conditions. In U.S. Pat. No. 4,958,512, the frictional measurement device can be set up to “seek out” the slip factor which will provide the maximal runway friction, and measure the magnitude of that force. Alternately, this invention can be set up to measure the friction at a given fixed slip ratio anywhere from 0 to 100% slip. While this would be beneficial to a braking system which incorporated this idea, it is of limited use to current aircraft braking systems as they do not have the capabilities to ‘seek out’ and then maintain the maximum runway friction wheel slip ratio.
Finally, a device such as U.S. Pat. No. 7,617,721 can be used that more accurately mimics the stopping characteristics of an aircraft, so that more accurate calculations of stopping distances can be made by a pilot landing or taking off on a runway covered in snow, ice, or some other contaminant. With this device, a pilot will have a reasonable idea of how their aircraft will stop on that runway. However, all of the foregoing techniques rely upon ground crew to take active measurement steps before landing to analyze and approximate the conditions on each runway at the airport. Thus, it would be beneficial to provide instantaneous and dynamic information in real-time to the pilot of an actual aircraft that is performing a landing procedure.